1. Field of the Invention
This invention relates to an electromagnet, and in particular an electromagnet for operating a valve of the type comprising an armature which is movable between two limit positions and moves an object, for example the valve member, a core against which the armature moves when the coil is energised, the armature being guided by the body of the coil, and a spring for returning the armature.
2. Description of the Prior Art
Electromagnets of the kind described are used for example, although not exclusively, for operating a valve, such as a hydraulic or pneumatic valve.
The electromagnet can be incorporated with the valve in a structural unit, but it is possible to arrange the magnet separately and to operate the valve through some suitable element.
It is desirable in many cases in using an electromagnet of this kind to be able to achieve high frequence switching rates, for example rates of up to 9-digit numbers are necessary.
It is known to increase the effective life of a magnet by enclosing it movably in a guide tube of plastics material, in which case the guide tube may preferably serve simultaneously as a coil-carrying body. A further means for increasing the useful life lies in controlling the limits to the travel of the armature so as to keep mechanical strains on the latter as small as possible. Thus should the armature impact hard against the means limiting it there will sooner or later be some deformation of the impacting face of this armature and the formation of cracks is possible. The guide surfaces, particularly those of the armature guide tube, become damaged and this frequently is a cause of a reduced life.
It is known to use buffer means which dampens the impact of the armature more particularly on the pole face of the core. Resilient buffer discs and similar means have been used for this purpose. The introduction of such means is however difficult because frequently the air gaps which reduce the magnetic forces are increased thereby. To maintain the magnetic forces it is then necessary to increase the dimensions of the magnet, which means extra expense and drawbacks.
A corresponding increase in the dimensions means an enlargement of the mass of the armature which reduces the speed of travel of the armature and further increases the bulk of the armature to be dampened.
If the resilient buffer members are made comparatively small their effect is likewise small and the dampening means are thus overloaded and become prematurely worn.